Abstract
Plasmodium falciparum malaria is brought about by the asexual stages of the parasite residing in human red blood cells (RBC). Contact between the erythrocyte surface and the merozoite is the first step for successful invasion and proliferation of the parasite. A number of different pathways utilised by the parasite to adhere and invade the host RBC have been characterized, but the complete biology of this process remains elusive. We here report the identification of an open reading frame (ORF) representing a hitherto unknown second exon of the Pf332 gene that encodes a cysteine-rich polypeptide with a high degree of similarity to the Duffy-binding-like (DBL) domain of the erythrocyte-binding-ligand (EBL) family. The sequence of this DBL-domain is conserved and expressed in all parasite clones/strains investigated. In addition, the expression level of Pf332 correlates with proliferation efficiency of the parasites in vitro. Antibodies raised against the DBL-domain are able to reduce the invasion efficiency of different parasite clones/strains. Analysis of the DBL-domain revealed its ability to bind to uninfected human RBC, and moreover demonstrated association with the iRBC surface. Thus, Pf332 is a molecule with a potential role to support merozoite invasion. Due to the high level of conservation in sequence, the novel DBL-domain of Pf332 is of possible importance for development of novel anti-malaria drugs and vaccines.
Highlights
The invasion of red blood cells (RBC) by a Plasmodium-merozoite is a cascade like process involving adhesion, reorientation, junction-formation and invagination
During a survey to identify open reading frames (ORF) containing domains able to bind to RBC, the ORF PF11_0506 was identified, for which the adjacent ORF PF11_0507 encoding Pf332 started only 280 bp after the predicted stop codon of PF11_0506
Numerous in vitro studies have concluded that the cascade of the rapid invasion process, taking less than 20 seconds [26], is complex and involves a large number of different molecules
Summary
The invasion of RBC by a Plasmodium-merozoite is a cascade like process involving adhesion, reorientation, junction-formation and invagination. The parasite forms a tight junction with the RBC membrane involving several proteins discharged from micronemes and rhoptries in the apical part of the cell Among these are the erythrocyte binding proteins of the EBL-family located in the micronemes and the reticulocyte binding like (RBL) proteins situated at the neck of the rhoptries, which are expressed simultaneously by the merozoite. It is probably the available receptor on the RBC surface that determines which ligand the parasite employs for invasion [8]
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